Depends on what you're expecting. if you want to read a very well written book, go for Gibbon. He had style, wit, and intelligence, and he managed all the classical sources that were available by his time. But as for real knowledge, what you'll learn is the basic sequence of events of what happened (as he quoted them from the classical sources) and also an understanding on how the Roman Empire was seen in the Age of Enlightenment and how its demise was used in the political and ideological issues of the late XVIII century (for example, the appearance of modern biblical criticism and the first stances of contemporary atheism, hence its take on the influence of Christian religion on the fall of the empire).
If you want more objective, detailed and accurate information about the realities of the Roman empire during the 180 - 630s AD period, you'll need to read contemporary research. Modern books on the subject are far drier than Gibbon's though, as today a professional historian is expected to be far more meticulous and objective than Gibbon was. If you have to constantly declare what your sources are and justify all the facts and hypotheses you present, it's practically impossible that you'll write the kind of book that Gibbon wrote. But one thing is good prose, and another thing is good history. And in a history book, I personally give far more value to the later, as long as the author writes with a minimum of readability. There are many aspects of the late Roman empire that are far better understood now than in Gibbon's time:
- Rome's neighbours, the "barbarian" peoples of the north and the Sassanian empire, of which Gibbon had only the barest of knowledges. For example, he had no access to the Arabic sources which described the late Sassanian empire, or to Pahlavi inscriptions like those by Shapur I at his commemorative rock reliefs of his victories against the Romans. He also had no idea of what "barbarian" societies were really like, as the classical authors themselves left very poor accounts on the matter, and in this field archaeology has been invaluable.
- Roman economic and social realities. For example, the archaeological excavations of near eastern Roman cities have revealed that the V-VI centuries were a period of great prosperity for Asia Minor, Syria and Egypt, with generalized population growth. That conflicts badly with Gibbon's views of a decadent "Byzantine" empire. Also, he totally ignored the socio-economic picture that modern scholars have of the western Roman society, with the issues of growing inequality, concentration of the property of land in fewer hands, generalized tax evasion, etc.
- We also have a far better understanding of the military realities of the late Roman empire, and how the technological and organizative advantadge that Rome once had over its "barbarian" neighbours had mostly dissipated by the III century AD. By the IV century, it can be argued that eastern societies, both sedentary and nomadic, were even more technologically advanced in mobile warfare than Romans were.
All that knowledge comes from new archaeological evidence: excavations of ancient sites all over the Roman world, epigraphy, numismatics, Egyptian papyrii (very important findings like the Nag Hammadi scripts, the papyriii from Aphrodito and Oxyrrinchus, etc) and also from literary sources that Gibbons ignored. Those sources can be direct like in the case of Arabic texts, or indirect, derived from philological study and literary criticism of ancient sources already known to Gibbons: for example, he accepted the writings of authors like Tacitus or Suetonius almost uncritically at face value, while nowadays we understand that (as every human writer) they had a political, moral and personal agenda of their own, and their writings should be subjected to criticism.
As for recommended readings, I want to endorse also Chris Wickham's work, which offers a well-written-, clear, objective and detailed sumamry of the late roman and post-roman world up until the Carolingian/Abbasid eras. Also "The Roman Empire at Bay, AD 180-394" by David S. Potter is a good book, but quite hard to read. Also, for the III century crisis, I'd recommend Pat Southern's "The Roman Empire from Severus to Constantine", a quite readable book. "The Mediterranean World in Late Antiquity, AD 395-700" by Averil Cameron elaborates on Brown's earlier book and is also an exhaustive study of the realities of that era and geographical space.
